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<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="boost_asio.reference.windows__basic_stream_handle.async_read_some"></a><a class="link" href="async_read_some.html" title="windows::basic_stream_handle::async_read_some">windows::basic_stream_handle::async_read_some</a>
</h4></div></div></div>
<p>
          <a class="indexterm" name="boost_asio.indexterm.windows__basic_stream_handle.async_read_some"></a> 
Start
          an asynchronous read.
        </p>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span>
    <span class="keyword">typename</span> <a class="link" href="../MutableBufferSequence.html" title="Mutable buffer sequence requirements">MutableBufferSequence</a><span class="special">,</span>
    <span class="keyword">typename</span> <a class="link" href="../ReadToken.html" title="Read token requirements">ReadToken</a> <span class="special">=</span> <span class="identifier">default_completion_token_t</span><span class="special">&lt;</span><span class="identifier">executor_type</span><span class="special">&gt;&gt;</span>
<a class="link" href="../asynchronous_operations.html#boost_asio.reference.asynchronous_operations.automatic_deduction_of_initiating_function_return_type"><span class="emphasis"><em>DEDUCED</em></span></a> <span class="identifier">async_read_some</span><span class="special">(</span>
    <span class="keyword">const</span> <span class="identifier">MutableBufferSequence</span> <span class="special">&amp;</span> <span class="identifier">buffers</span><span class="special">,</span>
    <span class="identifier">ReadToken</span> <span class="special">&amp;&amp;</span> <span class="identifier">token</span> <span class="special">=</span> <span class="identifier">default_completion_token_t</span><span class="special">&lt;</span> <span class="identifier">executor_type</span> <span class="special">&gt;());</span>
</pre>
<p>
          This function is used to asynchronously read data from the stream handle.
          It is an initiating function for an <a class="link" href="../../overview/model/async_ops.html" title="Asynchronous Operations">asynchronous
          operation</a>, and always returns immediately.
        </p>
<h6>
<a name="boost_asio.reference.windows__basic_stream_handle.async_read_some.h0"></a>
          <span class="phrase"><a name="boost_asio.reference.windows__basic_stream_handle.async_read_some.parameters"></a></span><a class="link" href="async_read_some.html#boost_asio.reference.windows__basic_stream_handle.async_read_some.parameters">Parameters</a>
        </h6>
<div class="variablelist">
<p class="title"><b></b></p>
<dl class="variablelist">
<dt><span class="term">buffers</span></dt>
<dd><p>
                One or more buffers into which the data will be read. Although the
                buffers object may be copied as necessary, ownership of the underlying
                memory blocks is retained by the caller, which must guarantee that
                they remain valid until the completion handler is called.
              </p></dd>
<dt><span class="term">token</span></dt>
<dd>
<p>
                The <a class="link" href="../../overview/model/completion_tokens.html" title="Completion Tokens">completion
                token</a> that will be used to produce a completion handler, which
                will be called when the read completes. Potential completion tokens
                include <a class="link" href="../use_future.html" title="use_future"><code class="computeroutput"><span class="identifier">use_future</span></code></a>, <a class="link" href="../use_awaitable.html" title="use_awaitable"><code class="computeroutput"><span class="identifier">use_awaitable</span></code></a>, <a class="link" href="../yield_context.html" title="yield_context"><code class="computeroutput"><span class="identifier">yield_context</span></code></a>, or a function
                object with the correct completion signature. The function signature
                of the completion handler must be:
              </p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">handler</span><span class="special">(</span>
  <span class="keyword">const</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">system</span><span class="special">::</span><span class="identifier">error_code</span><span class="special">&amp;</span> <span class="identifier">error</span><span class="special">,</span> <span class="comment">// Result of operation.</span>
  <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">bytes_transferred</span> <span class="comment">// Number of bytes read.</span>
<span class="special">);</span>
</pre>
<p>
                Regardless of whether the asynchronous operation completes immediately
                or not, the completion handler will not be invoked from within this
                function. On immediate completion, invocation of the handler will
                be performed in a manner equivalent to using <a class="link" href="../async_immediate.html" title="async_immediate"><code class="computeroutput"><span class="identifier">async_immediate</span></code></a>.
              </p>
</dd>
</dl>
</div>
<h6>
<a name="boost_asio.reference.windows__basic_stream_handle.async_read_some.h1"></a>
          <span class="phrase"><a name="boost_asio.reference.windows__basic_stream_handle.async_read_some.completion_signature"></a></span><a class="link" href="async_read_some.html#boost_asio.reference.windows__basic_stream_handle.async_read_some.completion_signature">Completion
          Signature</a>
        </h6>
<pre class="programlisting"><span class="keyword">void</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">system</span><span class="special">::</span><span class="identifier">error_code</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">)</span>
</pre>
<h6>
<a name="boost_asio.reference.windows__basic_stream_handle.async_read_some.h2"></a>
          <span class="phrase"><a name="boost_asio.reference.windows__basic_stream_handle.async_read_some.remarks"></a></span><a class="link" href="async_read_some.html#boost_asio.reference.windows__basic_stream_handle.async_read_some.remarks">Remarks</a>
        </h6>
<p>
          The read operation may not read all of the requested number of bytes. Consider
          using the <a class="link" href="../async_read.html" title="async_read"><code class="computeroutput"><span class="identifier">async_read</span></code></a> function if you need
          to ensure that the requested amount of data is read before the asynchronous
          operation completes.
        </p>
<h6>
<a name="boost_asio.reference.windows__basic_stream_handle.async_read_some.h3"></a>
          <span class="phrase"><a name="boost_asio.reference.windows__basic_stream_handle.async_read_some.example"></a></span><a class="link" href="async_read_some.html#boost_asio.reference.windows__basic_stream_handle.async_read_some.example">Example</a>
        </h6>
<p>
          To read into a single data buffer use the <a class="link" href="../buffer.html" title="buffer"><code class="computeroutput"><span class="identifier">buffer</span></code></a> function as follows:
        </p>
<pre class="programlisting"><span class="identifier">handle</span><span class="special">.</span><span class="identifier">async_read_some</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">buffer</span><span class="special">(</span><span class="identifier">data</span><span class="special">,</span> <span class="identifier">size</span><span class="special">),</span> <span class="identifier">handler</span><span class="special">);</span>
</pre>
<p>
          See the <a class="link" href="../buffer.html" title="buffer"><code class="computeroutput"><span class="identifier">buffer</span></code></a>
          documentation for information on reading into multiple buffers in one go,
          and how to use it with arrays, boost::array or std::vector.
        </p>
<h6>
<a name="boost_asio.reference.windows__basic_stream_handle.async_read_some.h4"></a>
          <span class="phrase"><a name="boost_asio.reference.windows__basic_stream_handle.async_read_some.per_operation_cancellation"></a></span><a class="link" href="async_read_some.html#boost_asio.reference.windows__basic_stream_handle.async_read_some.per_operation_cancellation">Per-Operation
          Cancellation</a>
        </h6>
<p>
          This asynchronous operation supports cancellation for the following <a class="link" href="../cancellation_type.html" title="cancellation_type"><code class="computeroutput"><span class="identifier">cancellation_type</span></code></a>
          values:
        </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
              <code class="computeroutput"><span class="identifier">cancellation_type</span><span class="special">::</span><span class="identifier">terminal</span></code>
            </li>
<li class="listitem">
              <code class="computeroutput"><span class="identifier">cancellation_type</span><span class="special">::</span><span class="identifier">partial</span></code>
            </li>
<li class="listitem">
              <code class="computeroutput"><span class="identifier">cancellation_type</span><span class="special">::</span><span class="identifier">total</span></code>
            </li>
</ul></div>
</div>
<div class="copyright-footer">Copyright © 2003-2025 Christopher M.
      Kohlhoff<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
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